The lunula is the pale half-moon at the base of a fingernail. When it turns blue or slate gray while the rest of the nail bed stays normal, the list of explanations gets very short, and one entry on it is a metal that never leaves the body.
An 88-year-old man presented to a dermatology team at the University of Oklahoma with exactly that pattern, unchanged for 50 years. The discoloration covered the proximal fingernails of both hands and stopped at the edge of the lunulae. There was no nail plate damage, no toenail involvement, and no other skin or mucous membrane findings. On questioning, he disclosed a lifetime history of colloidal silver use, applied to the skin as an antiseptic and used in the mouth for canker sores.
